Phages of dairy Leuconostoc mesenteroides: genomics and factors influencing their adsorption.

Phages infecting Leuconostoc mesenteroides strains can be overlooked during milk fermentation because they do not slowdown the acidification process. However, they can negatively impact the flavor profile of the final product. Yet, the information about these phages is still scarce. In this work, we investigated diverse factors influencing the adsorption of seven virulent Ln. mesenteroides phages, isolated from blue cheese manufacture in Argentina, to their host cells. The addition of calcium ions was generally necessary to observe complete cell lysis and plaque formation for four of the seven phages, but adsorption was very high even in the absence of this cation for all phages. The temperature barely influenced the adsorption process as it was high within the temperature range tested (0 to 50 °C). Moreover, the kinetics of adsorption were similar on viable and non-viable cells, revealing that phage adsorption does not depend on physiological state of the bacterial cells. The adsorption rates were also high at pH values from 4 to 9 for all Ln. mesenteroides phages. We also analyzed the complete genome sequences of two of these phages. Complete nucleotide analysis of phages Ln-8 and Ln-9 showed dsDNA genomes with sizes of 28.5 and 28.9 kb, and the presence of 45 and 48 open reading frames (ORFs), respectively. These genomes were highly similar to those of previously characterized Φ1-A4 (USA, sauerkraut, fermentation) and ΦLN25 (England, whey), both virulent Ln. mesenteroides phages. A detailed understanding of these phages will lead to better control strategies.

Review: efficiency of physical and chemical treatments on the inactivation of dairy bacteriophages.

Bacteriophages can cause great economic losses due to fermentation failure in dairy plants. Hence, physical and chemical treatments of raw material and/or equipment are mandatory to maintain phage levels as low as possible. Regarding thermal treatments used to kill pathogenic bacteria or achieve longer shelf-life of dairy products, neither low temperature long time nor high temperature short time pasteurization were able to inactivate most lactic acid bacteria (LAB) phages. Even though most phages did not survive 90°C for 2 min, there were some that resisted 90°C for more than 15 min (conditions suggested by the International Dairy Federation, for complete phage destruction). Among biocides tested, ethanol showed variable effectiveness in phage inactivation, since only phages infecting dairy cocci and Lactobacillus helveticus were reasonably inactivated by this alcohol, whereas isopropanol was in all cases highly ineffective. In turn, peracetic acid has consistently proved to be very fast and efficient to inactivate dairy phages, whereas efficiency of sodium hypochlorite was variable, even among different phages infecting the same LAB species. Both alkaline chloride foam and ethoxylated non-ylphenol with phosphoric acid were remarkably efficient, trait probably related to their highly alkaline or acidic pH values in solution, respectively. Photocatalysis using UV light and TiO(2) has been recently reported as a feasible option to industrially inactivate phages infecting diverse LAB species. Processes involving high pressure were barely used for phage inactivation, but until now most studied phages revealed high resistance to these treatments. To conclude, and given the great phage diversity found on dairies, it is always advisable to combine different anti-phage treatments (biocides, heat, high pressure, photocatalysis), rather than using them separately at extreme conditions.